Stretch-forming machine with stress-isolated base

ABSTRACT

A machine for stretch forming of sheet-metal parts, such as leading-edge surfaces for an aircraft wing, over an upright forming die. Gripping jaws are pivotally supported by frames on opposite sides of the die to rotate about pivot axes substantially parallel to side surfaces of the die. The jaws and frames are adjustable in position with respect to each other to accommodate dies which are tapered or have unequal side surfaces. Hydraulic cylinders drive the jaws from a loading position where a metal sheet is guided into the jaws, into a stretch-forming position where the jaws are tangent to the die surfaces and the sheet extends from the die through a right-angle bend into the jaws. A single cylinder is used on each jaw to accomplish initial positioning as well as to apply stretch-forming force. The die is supported on the upright jaw-supporting frames, and reaction forces arising during stretch forming are isolated from a base which carries the frames.

United States Patent [72] lnventor Landon R. Gray Portuguese Bend, Calif.

[21 App]. No. 747,598

[22] Filed July 25, 1968 [45} Patented Apr. 13, 1971 [73] Assignee Sheridan-Gray, lnc.

Huntington Park, Calif.

[54] STRETCH-FORMING MACHINE WITH STRESS- ISOLATED BASE 16 Claims, 10 Drawing Figs. [52] U.S. Cl 72/302, 72/31 1 [51] lnt.Cl B2ld 11/02 [50] Field of Search 72/302,

[56] References Cited UNITED STATES PATENTS 2,986,194 5/1961 DeMarco 72/297 2,702,929 3/1955 Laddon et al 24/263 2,753,915 7/1956 Raynes 72/305 2,835,947 5/1958 Gray et al. 24/81 Primary Examiner-Charles W. Lanham Assistant Examiner-Michael J. Keenan Att0rney-Christie, Parker & Hale ABSTRACT: A machine for stretch forming of sheet-metal parts, such as leading-edge surfaces for an aircraft wing, over an upright forming die. Gripping jaws are pivotally supported by frames on opposite sides of the die to rotate about pivot axes substantially parallel to side surfaces of the die. The jaws and frames are adjustable in position with respect to each other to accommodate dies which are tapered or have unequal side surfaces. Hydraulic cylinders drive the jaws from a loading position where a metal sheet is guided into the jaws, into a stretch-forming position where the jaws are tangent to the die surfaces and the sheet extends from the die through a right-angle bend into the jaws. A single cylinder is used on each jaw to accomplish initial positioning as well as to apply stretch-forming force. The die is supported on the upright jawsupporting frames, and reaction forces arising during stretch forming are isolated from a base which carries the frames.

s Sheets-Sheet 1 Patented April 13, 1971 6 Sheets-Sheet 2 INVENTOR.

"Pate ted April 13, 1911 6 Sheets-Sheet 5 Pateht ed April 13, 1971 3,575,031

6 Sheets-Sheet 4 Pateiikd April 13, 1911 j 3,575,031

6 Sheets-Sheet 5 STRETCH-FORMING MACHINE WITH STRESS- ISOLATED BASE BACKGROUND OF INVENTION Stretch forming is a method of forming parts by stretching a metal sheet beyond its elastic limit over a die. The sheet is deformed by tension forces into a shape corresponding to the die, and retains this shape because it is stressed beyond its elastic limit. This method is widely used in the aircraft and automotive field for fabricating sheet-metal components, and is further described in US. Pat. Nos. 2,824,594, 2,835,947, 3,073,373 and 3,299,688.

An important use of stretch forming is in precision fabrication of sharply curved elongated sections such as leading-edge panels for aircraft wings. A metal blank, typically aluminum or steel sheet stock, is initially flat and usually has a generally rectangular shape. In conventional stretching machines, the blank is loaded into a pair of opposed gripping jaws which are then positioned by associated auxiliary hydraulic cylinders to an initial stretch-forming position. A die carried by a main hydraulic cylinder is then driven against the blank with sufficient force that the blank is stretched beyond its elastic limit over the die. The blank is thereby formed into a desired shape which corresponds to the shape of the die.

Conventional stretching machines perform these operations successfully, but these machines are characterized by several inherent drawbacks. The use of separate hydraulic cylinders (or equivalent drive means) for jaw positioning and for application of primary stretching forces creates a difficult control problem which complicates the design anduse of the machines and makes them expensive to construct. This is because the ultimate force delivered to the part is dependent on the relative positioning of the jaws and the die, and the positioning of these components by a multiplicity of hydraulic cylinders must be established and maintained with precision by an elaborate control system and by the constant supervision of a highly skilled operator.

Conventional machines are also not well suited to economical production of parts having side surfaces which extend unequal distances from the upper edge or nose" of the die. For example, a wing leading-edge member may have an upper surface which extends rearwardly beyond the trailing edge of a lower surface of the member. Conventional machines are normally capable of forming these surfaces only with equal cordwise dimensions and the stretched blank must be trimmed to achieve the desired final shape. This results in a considerable loss of expensive sheet stock as the trimmings are usually not useful for other purposes.

A further problem with known machines is that the high reaction forces created during stretch forming are transmitted and carried by a base which mounts the gripping jaws, die, hydraulic cylinders and other components. The base must be of massive construction to withstand these loads, and the weight and cost of the machine is increased by the use of 'heavy rigid base beams. Such machines are also often awkward to load with an initially flat part which must be curved over the die before stretch forming begins.

The stretch-forming machine of this invention is free of the limitations of known machines as discussed above, and is capable of precise and reproducible fabrication of stretchforrned parts. The machine includes a pair of opposed, pivotally mounted gripping jaws supported by frames extending upwardly from a base onopposite sides of a stationary die. Each jaw uses only a single hydraulic directed for both initial jaw positioning and application of stretchfonning forces. Expense of the machine is thereby reduced, and operation is greatly simplified as the problem of controlling multiple cylinders to establish the relative positions of the die and jaw is avoided. That is, once an initial setup is made for a particular die, the geometry of the machine is fixed and stretch forming can then be carried out by a relatively unskilled operator and without any need for complex control systems.

The pivot axes of the jaws are horizontal and parallel to adjacent side surfaces of the die, and are movable with respect to each other to permit formation of nonuniform or tapering parts with a minimum of raw-material loss from trimming. .law clamping forces are reduced because reaction forces tending to pull the metal blank out of the jaws are directed approximately normally to a clamping plane of each jaw rather than directly into jaws. The die is carried directly by the jaw-supporting frames, and reaction forces are isolated within the frames and jaws such that the base can be made of relatively light beams or other members capable of supporting only the deadweight of the machine. Long dies are accommodated by coupling together a plurality of machines in modular fashion.

SUMMARY OF THE INVENTION Briefly stated, the machine of this invention is useful for transverse stretch forming of a metal sheet, and includes a base and a pair of spaced-apart upright frames mounted on the base to be movable horizontally and vertically with respect to each other for initial setup positioning. An elongated die is positioned and supported by the frames out of direct contact with the base to be stationary during a stretch-forming operation. A pair of gripping jaws are pivotally secured to the frames on opposite sides of the die for clamping engagement with opposite longitudinal margins of the metal sheet. Each gripping jawhas coupled to it a primary power means, such as a hydraulic cylinder, which is operable to rotate the jaw downwardly from a raised sheet-loading position to a stretchforming position in which the respective jaws are disposed adjacent opposed side surfaces of the die and are positioned to stretch form the metal sheet. Preferably, one of the frames is movable about an upright axis so it can be aligned substantially parallel with the adjacent side surface of a tapering die.

The machine is characterized by a fixed kinematic geometry after an initial setup for a specific die has been made. Each gripping jaw includes a secondary power means, such as a hydraulic cylinder, to provide clamping force for engaging the jaw with the margin of the sheet. The secondary power means is independent of the primary power means and has no effect on the relative positioning of the die and respective jaw when the jaw is clamped in engagement with the sheet. Preferably, the machine further includes a sheet-loading means adapted to receive and hold an initially flat metal sheet which is to be stretch formed. The loading means is operable to curve the sheet such that opposite margins thereof are aligned with clamping planes of the jaws when the jaws are in a raised loading position.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in detail with reference to the attached drawings, in which:

FIG. I is an elevation, partly broken away, of a stretchforming machine according to the invention, with gripping jaws in a raised sheet-loading position;

FIG. 2 is an end view, partly broken away, on line 2-2 of FIG. 1;

FIG. 3 is a side elevation similar to FIG. I, but with the gripping jaws in a lowered stretch-fonning position;

FIG. 4 is a top view, partly broken away, of the machine with one jaw assembly in a raised loading position and another jaw assembly in a lowered stretch-forming position;

FIG. 5 is a sectional view on line 5-5 of FIG. 3;

FIG. 6 is a partial end view of the machine on line 6-6 of FIG. I;

FIG. 7 is a schematic representation of a top view of a plurality of stretch-forming machines coupled together in modular fashion for use with an elongated and tapered die, and showing a sheet-loading device in phantom line;

FIG. 8 is a side elevation of a stretch-forming machine with a sheet-loading device;

FIG. 9 is a sectional view on line 9-9 of FIG. 8; and

FIG. is a sectional view on line 10-10 of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 14, a stretch-forming machine I0 according to the invention has a base 11 which rests on a concrete foundation 12 forming a pit below the level of a surrounding floor 13. The base includes a pair of elongated and laterally spaced upright beams 14 secured together at their lower ends by a baseplate 15. An intermediate upright cross member 16 is also secured to the beams and the baseplate to provide additional rigidity in the base, and end plates (not shown) span the beams for the same purpose.

An elongated horizontal plate or upper rail 17 is secured to the upper surface of each beam 14 so the beam and rail have a generally T-shaped cross section. A pair of lower rails l8 are secured to opposite sides of each beam 14 and extend horizontally from the right end (as seen in FIGS. I and 3) part way along the length of the beam to define horizontal channels 19. A pair of vertical plates 20 are secured to opposite sides of each beam I4, and are spaced longitudinally from the right end of the beam. Each plate 20 has a vertical channel 21 formed therein, and upper rails I7 are notched such that channels 21 are continuous to the top of the base.

A vertically movable upright frame 25 is positioned between upright beams I4 and is supported by the base. Frame 25 includes a pair of spaced-apart upright sideplates 26 which are secured together by a rigid web 27. Each sideplate 26 has a vertical rail 28 secured thereto, and these rails mate with inner vertical channels 21 in the base to guide frame 25 as it is moved vertically by a supportingjackscrew 30 which is threaded through a mating coupling 31 secured to baseplate 15. The jackscrew is rotated by a conventional electric or hydraulic power means 32 or alternatively can be manually rotated.

The machine is designed so various forces arising during a stretch-forming operation are not carried by jackscrew 30, and the jackscrew need therefore only support the deadweight of the associated upright frame and the components mounted on the frame. In most applications, it is satisfactory to use the jackscrew as the sole support for the upright frame, and the frame is thereby made vertically positionable at any desired position as guided by vertical rails 28 in vertical channels 2].

In large machines, additional anchoring of the upright frame to the base may be desired, and this is readily accomplished by providing a plurality of vertically spaced horizontal channels 33 in the outer faces of sideplates 26. The upright frame is then driven by the jackscrew to a position which is close to the desired frame vertical position and which places pairs of channels 33 in the frame and channels 19 in the base in vertical alignment. A pair of locking bars 34 of square cross section are then inserted in the opposed channels to lock together the frame and base. If the stepwise vertical adjust ment provided by the use of the locking bars does not provide small enough increments of vertical positioning, an infinite number of vertical positions can be provided without locking bars by using a very heavy jackscrew or several jackscrews to support the frame.

A horizontally movable upright frame 37 is carried by and positioned between upright beams 14 of the base. Frame 37 includes a pair of upright sideplates 38 which are secured together by a web 39. A horizontal guide channel 40 is formed in the outer face and adjacent the lower edge of each sideplate 38. Upper rails 17 of the base mate with guide channels 40 so frame 37 can slide horizontally along the top of the base toward and away from frame 25. Preferably, frame 37 is driven horizontally along the base by a pair of conventional power-operated horizontal jackscrews 41 secured to the frame and to foundation I2 as shown in FIGS. 1 and 4, or alternatively to base ll.

Preferably, one of the upright frames is adapted to be rotatable about a vertical axis within a limited range of say plus or minus 5. This rotational or skew adjustment permits the machine to be used with dies having a tapering cross section. A suitable way of providing this adjustment is to form horizontal guide channels 40 sufficiently deeply in sideplates 38 that a clearance space 43 (see FIG. 6) exists between the bottom of the channels and the innner edges of upper rails 17 which extend into the channels. This clearance space permits frame 37 to be rotated slightly about a vertical axis without upper rails 17 becoming disengaged from horizontal guide channels 40.

Preferably, horizontal jackscrews 41 are pivotally secured to sideplates 38 and to the base or foundation so they can be differentially driven to rotate the frame around an upright axis. A pair of locking screws 44 are threaded through the inside of sideplates 38 to extend horizontally into channels 40 to bear on the inner vertical faces of upper rails 17. The locking screws are backed off out of contact with the rails when jackscrews 41 are differentially actuated to rotate the frame to a desired position, and the screws are then tightened against the rails to lock the frame in place.

An elongated upright die 50 is positioned between and carried solely by upright frames 25 and 37. Die 50 may of course have a variety of different shapes, and is shown with a cross section as might be used to form a leading-edge member for an aircraft wing. The die has a forming surface 51 which extends downwardly from opposite sides of an upper edge 52 to terminate at end-of-die lines or parting lines 53 which mark the ultimate edges of the part to be stretch formed. Each side face of the die is concavely curved inwardly beneath the parting line to define ajaw-clearance portion 54. Each side of the die is inwardly stepped at a lower end of the jaw-clearance portion to define a pair of horizontal support surfaces 55 extending along the length of the die.

A die-supporting bracket 57 is rigidly secured to each of sideplates 38 in upright frame 37, and the brackets extend toward the die such that horizontal support surface 55 of the die rests on the upper surfaces of the brackets. A pair of diesupporting brackets 58 are rigidly secured to each sideplate 26 of frame 25, and these brackets also extend toward the die under horizontal support surface 55 at the right side (as seen in FIGS. 1 and 3) of the die. As best seen in FIG. 4, brackets 57 and 58 are formed so that they may mesh together in tongue-and-groove fashion to permit horizontally movable frame 37 to be moved closely adjacent vertically moveable frame 25 when a die of very narrow cross section is used.

Die 50 is thus supported entirely by upright frames 25 and 37, and does not directly contact base I I of the machine. This construction is advantageous because the relatively high-level stretching forces and the resulting reaction forces are confined entirely within the upright frames, die and grippingjaw assemblies. That is, the large forces which arise during a stretch-forming operation are isolated from the base, and the base need only be sturdy enough to carry the deadweight of the die, frames and jaws. This configuration permits a substantial cost reduction in the machine as' fewer structural members are needed in the base, and lighter-gauge materials can be used.

A gripping-jaw assembly 65 is pivotally mounted at the upper end of each of upright frames 25 and 37 by a large cylindrical pivot pin 66 (of say 4% inches diameter) which is journaled through assembly 65 and upper ends 67 of sideplates 26 and 38 of the frames. Assemblies 65 are identical in construction, and are mounted on the frames to hinge downwardly around the axes of pins 66 and toward each other on opposite sides of die 50 when the machine is in operation. Each gripping-jaw assembly includes one or more clamping units 68, and each assembly 65 as shown in the drawings has three such clamping units. The number of clamping units needed is determined by the clamping force required in a particular stretch-forming operation, and one or two units will be adequate when stretching light-gauge materials.

Each clamping unit 68 includes a pair of upperjaw plates 70 and a pair of lower jaw plates 71 sandwiched between plates 70. As best seen in FIG. 4, the middle clamping unit shares the upper jaw plates on the inner sides of the two outboard clamping units, and a total of four upper jaw plates is therefore adequate to assemble three clamping units.

Lower jaw plates 71 of the outboard clamping units are positioned against and on opposite sides of upper ends 67 of sideplates 26 and 38 of the upright frames. The lower jaw plates of the middle clamping units are spaced apart by a spacing ring 72 to be in face-toface contact with the inner surfaces of the associated upper jaw plates. Pivot pin 66 is journaled through all of the upper and lower jaw plates and spacing ring 72, and the jaw plates pivot freely about the pin.

Each jaw assembly has coupled thereto primary power means such as a hydraulic cylinder 75 which is pivotally mounted by a pair of trunnions 76 journaled through sideplates 26 and 38 of the respective upright frames. In a typical stretch-forming machine, cylinder 75 will develop about 250 tons of force. Each cylinder 75 has a piston rod 77, and a connecting yoke 78 is formed at the upper end of the piston rod. The connecting yoke has a plurality of upwardly extending fingers 79 which are interleaved between clamping units 68 to fit snugly against the inner surfaces of lower jaw plates 71.

A second pivot pin 82 is journaled through close-fitting bores in fingers 79 of the connecting yoke, and this pin is also journaled through close-fitting bores'in the free ends of all upper jaw plates 70. Pivot pin 82 has an axis which is parallel to the axes of pin 66 and trunnions 76. An elongated clearance slot 83 is formed in the freeend of each of lower jaw plates 71, and pivot pin 82 also passes'through these slots. The upper jaw plates are thus pivotally connected to piston rod 77, and these plates are constrained by pins 66 and 82 to move together as a unit when driven by hydraulic cylinder 75 and piston rod 77.

Each clamping unit 68 includes a power clamping means such as a clamping hydraulic cylinder 84 which typically develops about 70 tons of force. Each cylinder 84 is pivotally mounted between a respective pair of lower jaw plates 71 by a pair of trunnions 85 journaled through the lower jaw plates. Each clamping cylinder further has a piston rod 86 with a bored head 87 through which second pivot pin 82 is journaled.

Upper jaw plates 70 are shaped substantially as circular sectors with a circumferential surface 89 having an elongated notch 90 formed therein. An upper clamping bar 91 is fitted in notches 90 and is rigidly secured to all of the upper jaw plates in'each gripping-jaw assembly. The upper clamping bar thus extends across the full length of each gripping-jaw assembly, and further rigidly couples together the respective upper jaw plates in each assembly. A serrated gripping member 92 is rigidly secured to upper clamping bar 91 and extends along the full length of the clamping bar.

The upper end of each of lower jaw plates 71 has a notch 94 formed therein, and a lower clamping bar 95 is positioned innotches 94 and is rigidly secured to each of the lower jaw plates. The lower clamping bar thus extends across the full length of each respective gripping-jaw assembly, and rigidly secures together all of the lower jaw plates in that assembly. An elongated serrated gripping member 96 is rigidly secured to lower clamping bar 95 and is positioned in face-to-face relationship with gripping member 92.

Preferably, gripping members 92 and 96 have interfitting gripping teeth which nest together when the members are moved against each other. This hill and valley" grip provides a very secure hold on sheet stock which is to be stretch formed, and the gripping members are not damaged if they are inadvertently clamped together without a piece of sheet stock therebetween.

The jaws of a typical stretch-fon'ning machine as described above are about 40 inches long, and a single unit of the machine is thus useful with raw stock which is not more than about 40 inches in length. In many applications, however, considerably longer lengths of raw stock must be formed over a die 50 which may be feet orv more in length. In these situations, a plurality of machines 10 are ganged together to operate as a unit with the long sheet of raw stock. That is, each machine- 10 is useful by itself, or as a module in a larger assembly of such machines.

Such a multiple-module machine 100 is shown in schematic form in FIG. 7, and includes six modules 10A, 10B, 10C, 10D, 10E and 10F positioned side-by-side to accept sheet stock up to 20 feet in length. Each module includes a pair of upright frames 25 and 37 which carry associated gripping-jaw assemblies 65 as described above. Adjacent modules share upright beams 14 in that adjacent frames 25 are guided vertically in opposed vertical channels 21 on each beam, and adjacent frames 37 are guided horizontally on upper rail 17 of each shared beam. All of the upright frames 25 and 37 are accurately aligned so die 50 is equally supported by brackets 57 and 58 on all of the modules.

The several gripping-jaw assemblies can be coupled together to move in absolute synchronism by passing an elongated rigid locking bar (not shown) through holes 101 formed through the upper end of each of upper jaw plates 70. In the modular assemblies shown in FIG. 7, each of upright frames 37 has been skewed or rotated clockwise approximately 5 around its vertical axis whereby the coupled gripping-jaw assemblies on opposite sides of the die converge toward each other. A long tapering die 50A, such as used for forming aircraft-wing leading edges, is thus readily accommodated by the assembly of stretch-forming modules. The individual modules in machine 100 can also be operated separately with separate short dies, and in this case the adjacent gripping-jaw assemblies would of course not be locked together and could move independently of each other. This style of multimodule machine can also be constructed with the modules movably mounted on rails (not shown) so the modules can be moved toward or away from each other to be useful with dies of various lengths.

The operation of the machine is illustrated in FIG. 1 which shows the machine in a loading position, and in FIG. '3 which shows the machine in a stretch-forming position. Referring to FIG. I, gripping-jaw assemblies 65 are elevated upwardly around pivot pins 66 by cylinders 75 into a sheet-loading position. Each jaw has a clamping plane (the plane of a marginal side surface of a fiat sheet of stock clamped between gripping members 92 and 96, this plane being parallel to the axes of trunnions 76 and pivot pins 66 and 82) which is at a convenient loading angle of say 75 to the horizontal when the jaw assembly is in the loading position.

The gripping jaws of the machine are opened (as shown in the right-hand jaw in FIG. 1) by actuating clamping cylinders 84 to retract piston rods 86. Head 87 on each piston rod 86 is anchored to pivot pin 82, and a retracting actuation of cylinders 84 has the effect of drawing the cylinder itself toward pin 82. This in turn moves each lower jaw plate 71 downwardly (orcounterclockwise on the right-hand grippingjaw assembly in FIG. 1) with respect to the stationary upper jaw plates, with slot 83 providing clearance over pivot pin 82. That is, the lower plates rotate in scissors-fashion with respect to the upper plates when clamping piston rods 86 are retracted into respective cylinders and gripping members 92 and 96 and the associated clamping bars are thereby drawn apart to open the jaw. An advantage of this design is that the jaw opening can be very wide (a 4-inch spacing between gripping members is easily accomplished) to accept metal blanks having deformed edges, and to simplify release of the formed part.

A metal blank or sheet 105 is initially fiat and must be transversely arched or curved as shown in FIG. 1 to be loaded into the open gripping jaws. Although blanks or sheets for small parts can be manually curved and loaded into the jaws, most applications for this type of stretch-forming press involve the handling of relatively large and massive heavy-gauge sheets which must be handled and precurved with a poweractuated sheet-handling device 106 as partially shown in FIG. 1.

Loading means such as device 106 are commercially available, and typically include a horizontal beam 107 suspended from an overhead crane (not shown). A pair of arms 108 are pivotally secured to beam 107 to be movable by hydraulic cylinders 109 between a horizontal sheet-pickup position (shown in phantom in FIG. 1) and a loading position (shown in solid line in H6. 1) in which the arms extend approximately parallel to the clamping planes of respective gripping-jaw assembly 65. A plurality of suction cups 110 are mounted at the ends of spring-loaded plungers 111 spaced along beam 107 and arms 108, and the suction cups are coupled to a vacuum source (not shown).

In operation, sheet-handling device 106 is positioned by its supporting crane over an initially flat sheet with suction cups 110 resting against the upper surface of the sheet. The vacuum source is then actuated so the suction cups adhere tightly to the sheet, and the supporting crane elevates horizontal beam 107 and positions the metal sheet over die 50. Hydraulic cylinders 109 are then actuated to pivot arms 108 downwardly into the loading position shown in solid line in FIG. 1.

The downward motion of arms 108 forces the sheet into a curved configuration in which the side surfaces of the sheet are aligned with the clamping planes of the respective gripping-jaw assemblies. The sheet-handling device is then lowered by the crane until the longitudinal edges of the sheet margins bottom in the gripping-jaw assemblies and abut the upper surface of each upper jaw plate 70 in notch 90. Alternatively, adjustable stops (not shown) can be positioned in the upper jaw plates in notch 90 so the penetration of the sheet into the jaws can be adjustably controlled.

When sheet 105 is positioned in the open jaws as just described, hydraulic cylinders 84 are actuated to drive piston rods 86 and heads 87 away from the respective cylinders. Since each piston rod and head is anchored on pivot pin 82, the cylinder itselfis driven away from this pivot pin and carries with it lower jaw plate 71. Clamping bars 91 and 95 and associated gripping members 92 and 96 are thus moved into gripping engagement with opposite sides of the sheet as shown on the left-hand assembly 65 in P10. 1. The serrated teeth on the gripping members penetrate the surface of the sheet and hold it securely in the jaws. Pressure is maintained on cylinders 84 throughout the stretch-forming operation to lock the sheet in the closed jaws. Due to the difference in effective areas of an internal piston (not shown) in clamping cylinder 84, a larger force is available from clamping cylinders 84 when the cylinder is propelling the piston rod away from the cylinders 84 when the cylinder is propelling the piston rod away from the cylinder rather than retracting the piston rod. The described orientation of the clamping cylinder and jaw plates is therefore preferred as maximum force is needed when the jaws are in the closed position, and only a small amount of force is required to open the jaws.

After the curved sheet has been locked in the grippingjaws of the machine, suction cups 110 of sheet-handling device 106 are released, and main hydraulic cylinders 75 are actuated to retract piston rods 77 downwardly into the cylinders. This action forces the gripping-jaw assemblies to rotate downwardly around pivot pin 66 into a stretch-forming position as shown in FIG. 3. The cylinders are actuated with sufficient pressure not only to draw the sheet tightly over die 50, but also to elongate the sheet transversely beyond its elastic limit over the die in accordance with conventional stretch-forming practice. Stretching the sheet beyond its elastic limit produces a finished part which conforms accurately to the contour of the die, and which does not tend to spring back toward its original flat shape.

When a die such as die 50 having unequal side surfaces is used, piston rods 77 of the two hydraulic cylinders will be retracted at different rates to provide the same percentage of elongation of the metal sheet on each side of the die. Precise control over the actuation of the primary-power cylinders can be provided by using conventional control devices which are commercially available. For brevity, these devices will not be described in detail.

The downward travel of the gripping-jaw assemblies is limited by convention stops conventional stops (not shown), the position of which is determined during the initial setup of the machine for a specific die. Alternatively, electrical or hydraulic limit switches (not shown) can be used to release the actuating pressure on cylinder 75 when the sheet has been stressed a desired amount beyond its elastic limit. An

automatic control system, such as shown in my US. Pat. No.

2,824,594, can also be used to monitor and control the extent to which the sheet is stressed.

As shown in FIG. 3, the jaws draw the sheet into jawclearance portions 54 below the parting lines or end-ofdie lines on the die, and the sheet and curved jaws fit snugly against these clearance portions. This arrangement insures a very tight wrap of the sheet over the entire periphery of the die right up to the parting lines. That is, the sheet and gripping jaws are tangent to the die at the parting lines, and accurately formed parts are thereby provided.

The vertical adjustability of upright frame 25 permits the gripping-jaw assembly on this frame to be positioned such that the clamping plane of this jaw intersects the die immediately below parting line 53. The stretch-forming machine can thus be used with nonsymmetrical dies having unequal side surfaces, and the margin of the sheet below the die parting line is of minimum length so the amount of material wasted when the finished part is trimmed is very small.

After the stretch-forming operation is completed, the gripping jaws are opened by actuating cylinders 84 to retract the piston rods, and suction cups 110 of the sheet-handling device are again engaged with the upper surface of the finished part. Main cylinders 75 are actuated to rotate the gripping jaws upwardly to a position where the sheet-handling device can lift the finished part out of the jaws and move it to a position away from the stretch-forming machine. The only remaining step is to trim the margins of the formed sheet which extended below the parting lines of the die and into the gripping jaws.

lt should be noted that the clamping planes of the respective jaw assemblies are approximately perpendicular to the die surface when the jaws are in the stretch-forming position. That is, the sheet makes an approximately right-angle bend beneath the parting line of the die where it enters the jaws. Only a portion of the restoring force exerted by the sheet when it is tensioned is thus acting to pull the sheet out of the closed jaws. Clamping cylinders 84 can be somewhat reduced in size by this arrangement because a smaller clamping force is required than when the clamping plane of the jaw is aligned approximately parallel with the die surface at the parting lines. Heavy loads on cylinders 84 are also avoided because the primary tensioning loads from cylinders 75 are not directly applied to or transmitted through the clamping cylinders.

Gripping-jaw assembly 65 and associated hydraulic cylinder 75 are coupled together to act as a third-class lever. That is, the hydraulic cylinder force is applied to the gripping-jaw assembly between its hinge axis or fulcrum (pivot pin 66) and the applied load (the restoring force exerted by the sheet on the jaw during stretch forming) when the assembly is in the stretch-forming position.

An alternative form of a sheet-handling device 120 for loading machine is shown in FIGS. 810, and in phantom line in H0. 7. Device 120 includes a frame having a pair of channel-shaped anns 121 secured to opposite ends of main pivot pin 66 outboard of the gripping-jaw assemblies. As suggested in H6. 7, if a plurality of machines are grouped together in modular form to accommodate a long die 50A, arms 121 are secured to the outboard ends of the pivot pins in the end machines. A pair of opposed rails 122 are rigidly secured to each of arms 121, and a traveling block 123 is slidingly engaged with the rails to be guided longitudinally therealong.

A threaded lead screw 125 extends along the length of each of arms 121, and is supported at the free end of each arm by a bearing block 126. The opposite end of the lead screw adjacent pivot pin 66 is engaged with and supported by a leadscrew drive motor 127 of a conventional type. The lead screw is constrained against axial motion, but is rotatable by drive motor 127. The lead screw extends through a threaded opening in traveling block 123 such that the traveling block can be driven longitudinally along the arm when the lead screw is rotated.

A sheet-carrier bar 130 extends between arms 121, and is rotatably secured by a conventional swivel bushing 131 to a short shaft 132 secured to the innner side of travelling block 123. The sheet-carrier bar is generally shaped as an inverted T in cross section, and has a base 134 and a flange 13S extending perpendicularly from the base. The end of flange 135 remote from the base is curved to define a pusher end 136. As best seen in FIG. 10, the sheet-carrier bar is supported on swivel bushing 131 such that the bar is offset from the axis of rotation of the bushing around shaft 132. A plurality of finger holes 137 are formed through base 134 under overhanging pusher end 136.

Each arm 121 of the sheet-handling device has associated with it a hydraulic cylinder 140 to move the arm between a sheet-loading position (shown in solid line in FIG. 8) and a sheet-delivery position (shown in phantom line in FIG. 8). Cylinder 140 is pivotally secured at its lower end to base 11 by a clevis fitting 141. A piston rod 142 extends from the cylinder and is pivotally secured to arm 121 by a second clevis fitting 1413 fastened to the underside of the arm.

In operation arms 121 of the sheet-handling device are initially positioned by cylinders 140 such that flange 135 on sheet-carrier bar 130 is approximately aligned with the top of die 50 and the clamping plane of the gripping-jaw assembly on upright frame 25 as shown in FIG. 8. if a tapered die is used, bar 130 is adjusted by differential operation of motors 127 to be parallel to the die, and swivel bushings 131 permit this alignment. A sheet 145 of metal to be stretch formed is then slipped between the open jaws of the gripping-jaw assembly, and the jaws are locked to anchor the gripped margin of the sheet. Small sheets can be handled manually during this operation, or an auxiliary lifting device such as a crane can be used to manipulate larger and heavier sheets.

Drive motors 127 are then actuated to retract traveling blocks 123 until the free end of the sheet seats at the junction of base 134 and flange 135, as shown in FIG. 8. Arms 121 are then rotated downwardly by cylinders 140, and motors 127 are simultaneously actuated for further retraction of the traveling blocks and sheetcarrier bar. The lowering of the arms and retraction of the sheet-carrier bar cause the sheet to bow upwardly over the die and then downwardly toward the gripping-jaw assembly on upright frame 37 as shown in phantom in FIG. 8.

With arms 121 in the sheet-delivery position, the traveling blocks are moved until the sheet-carrier bar is directly over the open jaws of gripping-jaw assembly 65 on upright frame 37. The sheet-carrier bar is then rotated to withdraw base 134 from beneath the longitudinal edge of the sheet as shown in phantom in FIG. 10. When the bar is rotated, pusher end 136 presses the sheet away from the base, and the sheet is then easily slipped downwardly into the open jaws of the grippingjaw assembly. The sheet-carrier bar can be rotated either manually by placing the fingers through holes 137 in the bar base, or can be rotated by a conventional power drive (not shown). The gripping jaws are then closed to lock the sheet in position, and the arms are elevated out of the way during the stretch-formin g operation.

There has been described a novel stretch-forming machine adapted for stretch forming of a metal sheet over an elongated die. The machine is more economical to construct than known stretch-forming devices because the kinematic geometry of the machine is straightforward and direct, and the number of expensive forgings, hydraulic cylinders and moving parts is minimized. An important advantage of this construction is that the geometry of the machine is fixed once a setup is made for a particular die, and quantity production of precision parts is possible without any need for a highly skilled operator who is capable of monitoring and controlling the many degrees of freedom of motion which characterize machines of the prior art.

Another very significant feature of the machine is that the heavy action and reaction forces arising during stretch forming are isolated from the base which need have only sufficient structural rigidity to carry the deadweight of the die, jaws and jaw-supporting frames. Major cost savings are thus made possible as the base can be constructed from relatively light beams. The jaw and supporting-frame assemblies are well-suited for use as modules in a multipurpose stretchforming machine, and these assemblies can also be mounted to pivot about a horizontal axis if desired to provide another degree of freedom in the relative positioning of the jaws. Regardless of jaw orientation, the die is supported by the jaw frames out of contact with the base, and the base is thereby isolated from loads occurring during stretch forming of the metal sheet.

The machine has been described in a presently preferred form, but modifications and variations of the basic design will suggest themselves to those skilled in the art. It is intended that all such variations and modifications fall within the scope of the following claims which define the invention in detail.

lclaim:

1. A machine for stretch forming of an elongated metal sheet, comprising:

a base;

a pair of spaced-apart upright frames mounted on the base to be movable horizontally and vertically with respect to each other for setup positioning, each frame having support means for supporting a die;

an elongated die having opposed side surfaces, and being positioned between the frames and supported by the die support means to be stationary with respect to the frames during a stretch-forming operation;

a pair of gripping jaws pivotally secured to the frames on opposite sides of the die and adapted for clamping engagement with opposite longitudinal margins of the metal sheet, each jaw being rotatable about a pivot axis which is fixed with respect to the associated frame, the pivot axis and support means on each frame being stationary with respect to each other when the frame is moved with respect to the base during setup positioning; and

primary power means coupled to each respective frame and gripping jaw and operable to rotate the jaws downwardly from a raised sheet-loading position to a stretch-forming position in which the jaws are disposed adjacent the opposed side surfaces of the die and are positioned to stretch form the metal sheet;

the die support means holding the die out of direct contact with the base whereby stretch-forming forces are confined within the frames, die, sheet, gripping jaws and hydraulic cylinders, and such forces are isolated from the base.

2. The machine defined in claim '1 and further comprising secondary power means coupled between the base and the frames for adjusting the vertical position of one frame and the horizontal position of the other frame with respect to the base.

3. The machine defined in claim 1 in which one of the frames is movable about an upright axis whereby a horizontal pivot axis of the gripping jaw mounted on said one frame can be aligned substantially parallel with the adjacent side surface of the die.

4. The machine defined in claim 1 in which the primary power means comprises a pair of hydraulic cylinders one of which is mounted on each of the upright frames and coupled to the respective gripping jaw.

5. A machine for stretch forming a metal sheet, comprising:

a base;

a pair of spaced-apart upright frames supported on the base, at least one of the frames being movable with respect to the base for setup positioning, each frame having support means for supporting a die;

a die supported on the die support means between the frames and out of direct contact with the base, the die having opposite faces defining forming surfaces for the sheet, each forming surface terminating at an end-of-die line at its lower extremity;

pair of gripping jaws pivotally secured to the frames on opposite sides of the die and adapted for clamping engagement with opposite margins of the sheet, each jaw being rotatable about a hinge axis from a loading position to a stretch-forming position and having a pair of clamping members defining a clamping plane, the clamping planes being approximately parallel to the respective die forming surface in the loading position and being approximately normal to the adjacent die forming surfaces in the stretch-forming position so the sheet is bent at approximately a right angle where it enters each jaw in the stretch-forming position; and

primary power means coupled to each respective gripping jaw and operable to move the jaws between the loading and stretch-forming positions and to apply stretching forces to the sheet through the jaws;

the die, die support means, gripping jaws, and primary power means all being supported by the frame out of direct contact with the base whereby stretching forces and reaction forces are confined to the frames and said frame-supported components and are isolated from the base.

6. The machine defined in claim in which the gripping jaws are adjacent the forming surfaces of the die at the completion of a stretch-forming operation with the clamping planes intersecting the die below the endcf-die lines.

7. The machine defined in claim 6 in which each gripping jaw includes a secondary power means coupled to move the clamping members toward each other to a closed position against the margin of the sheet, the secondary power means being independent of the primary power means and having no effect on the relative positioning of the die and respective jaw when the clamping members are in the closed position.

8. The machine defined in claim 7 in which the primary power means comprises a pair of hydraulic cylinders, each of which is secured to a respective upright frame, each cylinder having a piston rod; and in which each jaw includes a pivot member connected thereto and to the piston rod to pivotally couple the jaw and piston rod, the pivot member having an axis substantially parallel to the hinge axis of the jaw.

9. The machine defined in claim 8 in which one of the clamping members in each jaw is stationary with respect to the axis of the associated pivot member.

10. The machine defined in claim 9 in which the hydraulic cylinders and jaws are coupled to act as third-class levers in the stretch-forming position.

11. The machine defined in claim 10 in which the upright frames are movable horizontally and vertically with respect to each other, and in which one of the frames is rotatable about an upright axis and with respect to the base within a range of at least about plus or minus 5 of a center position in which the two gripping jaws are substantially parallel.

12. The machine defined in claim 5 and further comprising a loading means adapted to receive and hold an initially flat metal sheet to be stretch formed, and to curve the sheet such that opposite margins of the sheet are aligned with the clamping planes of the jaws when the jaws are in the loading position.

13. The machine defined in claim 12 in which the loading means comprises a pair of arms pivotally secured to the machine to be rotatable about one of the jaw hinge axes, and a sheet-carrier bar coupled to the arms and arranged to be movable toward and away from said hinge axis, the sheetcarrier bar being adapted for releasable engagement with one margin of an initially flat sheet having another margin disposed between the clamping members of the jaw which rotates about said axis, whereby retraction of the bar toward said axis curves the sheet over the die to position said one margin in alignment with the clamping plane of the other jaw when the jaws are in the loading position.

14. The machine defined in claim 13 In which the sheetcarrier bar defines an elongated seat for receiving said one margin of the sheet, and in which the bar is pivotally coupled to the arms to be rotatable for dumping said one margin from the seat.

15. A machine for stretch forming a metal sheet over a die, comprising: a base; a pair of upright frames mounted on the base; each of the upright frames having a means for supporting the die in a fixed relation to the frames and having a gripping means adapted for clamping engagement with the metal sheet; at least one of the gripping means being movably mounted on the associated frame and having coupled thereto a power means connected between the gripping means and the frame for driving the movable jaw over a fixed path with respect to the frame to stretch the metal sheet over the die; the die, gripping means and power means being supported on the frames out of direct contact with the base whereby action and reaction forces occurring during stretch forming are isolated from the base; at least one of the frames and associated gripping means, die-supporting means, and power means being movable as a unit with respect to the base for setup positioning without affecting relative positioning of said one frame and said three means. 16. The machine defined in claim 17 and further comprising a second machine of identical definition, the machines being positioned adjacent each other, and means for securing together the adjacent gripping means of the several machines to move together when the associated power means are actuated.

PO'HM UNI'IEI) STA'IES PA'IEN'I OFFICE clsli'milc/n 1, 0 1* (,0 1111111.. 1 ION Patent, No. 31 Dated A ril l3, l97l lnventofls) Landon R. Gray It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 8 after "into" insert the Column 4, line 41 after "so' delete "that". Column 7, lines 42 delete "cylinders 84 when the cylinder and 43 is ropelling the piston rod away from the line 72 delete "conventional stops" (second occurrence) Column 12, line 48 "claim 17'' should read claim 15 (Claim 16) Signed and sealed this 26th day of October 1971.

(SEAL) Attest:

EDWARD M.FLETC HER,JR. ROBERT GOITSCHALK Attesting Officer Acting; Commissionerof Patent 

1. A machine for stretch forming of an elongated metal sheet, comprising: a base; a pair of spaced-apart upright frames mounted on the base to be movable horizontally and vertically with respect to each other for setup positioning, each frame having support means for supporting a die; an elongated die having opposed side surfaces, and being positioned between the frames and supported by the die support means to be stationary with respect to the frames during a stretch-forming operation; a pair of gripping jaws pivotally secured to the frames on opposite sides of the die and adapted for clamping engagement with opposite longitudinal margins of the metal sheet, each jaw being rotatable about a pivot axis which is fixed with respect to the associated frame, the pivot axis and support means on each frame being stationary with respect to each other when the frame is moved with respect to the base during setup positioning; and primary power means coupled to each respective frame and gripping jaw and operable to rotate the jaws downwardly from a raised sheet-loading position to a stretch-forming position in which the jaws are disposed adjacent the opposed side surfaces of the die and are positioned to stretch form the metal sheet; the die support means holding the die out of direct contact with the base whereby stretch-forming forces are confined within the frames, die, sheet, gripping jaws and hydraulic cylinders, and such forces are isolated from the base.
 2. The machine defined in claim 1 and further comprising secondary power means coupled between the base and the frames for adjusting the vertical position of one frame and the horizontal position of the other frame with respect to the base.
 3. The machine defined in claim 1 in which one of the frames is movable about an upright axis whereby a horizontal pivot axis of the gripping jaw mounted on said one frame can be aligned substantially parallel with the adjacent side surface of the die.
 4. The machine defined in claim 1 in which the primary power means comprises a pair of hydraulic cylinders one of which is mounted on each of the upright frames and coupled to the respective gripping jaw.
 5. A machine for stretch forming a metal sheet, comprising: a base; a pair of spaced-apart upright frames supported on the base, at least one of the frames being movable with respect to the base for setup positioning, each frame having support means for supporting a die; a die supported on the die support means between the frames and out of direct contact with the base, the die having opposite faces defining forming surfaces for the sheet, each forming surface terminating at an end-of-die line at its lower extremity; a pair of gripping jaws pivotally secured to the frames on opposite sides of the die and adapted for clamping engagement with opposite margins of the sheet, each jaw being rotatable about a hinge axis from a loading position to a stretch-forming position and having a pair of clamping members defining a clamping plane, the clamping planes being approximately parallel to the respective die forming surface in the loading position and being approximately normal to the adjacent die forming surfaces in the stretch-forming position so the sheet is bent at approximately a right angle where it enters each jaw in the stretch-forming position; and primary power means coupled to each respeCtive gripping jaw and operable to move the jaws between the loading and stretch-forming positions and to apply stretching forces to the sheet through the jaws; the die, die support means, gripping jaws, and primary power means all being supported by the frame out of direct contact with the base whereby stretching forces and reaction forces are confined to the frames and said frame-supported components and are isolated from the base.
 6. The machine defined in claim 5 in which the gripping jaws are adjacent the forming surfaces of the die at the completion of a stretch-forming operation with the clamping planes intersecting the die below the end-of-die lines.
 7. The machine defined in claim 6 in which each gripping jaw includes a secondary power means coupled to move the clamping members toward each other to a closed position against the margin of the sheet, the secondary power means being independent of the primary power means and having no effect on the relative positioning of the die and respective jaw when the clamping members are in the closed position.
 8. The machine defined in claim 7 in which the primary power means comprises a pair of hydraulic cylinders, each of which is secured to a respective upright frame, each cylinder having a piston rod; and in which each jaw includes a pivot member connected thereto and to the piston rod to pivotally couple the jaw and piston rod, the pivot member having an axis substantially parallel to the hinge axis of the jaw.
 9. The machine defined in claim 8 in which one of the clamping members in each jaw is stationary with respect to the axis of the associated pivot member.
 10. The machine defined in claim 9 in which the hydraulic cylinders and jaws are coupled to act as third-class levers in the stretch-forming position.
 11. The machine defined in claim 10 in which the upright frames are movable horizontally and vertically with respect to each other, and in which one of the frames is rotatable about an upright axis and with respect to the base within a range of at least about plus or minus 5* of a center position in which the two gripping jaws are substantially parallel.
 12. The machine defined in claim 5 and further comprising a loading means adapted to receive and hold an initially flat metal sheet to be stretch formed, and to curve the sheet such that opposite margins of the sheet are aligned with the clamping planes of the jaws when the jaws are in the loading position.
 13. The machine defined in claim 12 in which the loading means comprises a pair of arms pivotally secured to the machine to be rotatable about one of the jaw hinge axes, and a sheet-carrier bar coupled to the arms and arranged to be movable toward and away from said hinge axis, the sheet-carrier bar being adapted for releasable engagement with one margin of an initially flat sheet having another margin disposed between the clamping members of the jaw which rotates about said axis, whereby retraction of the bar toward said axis curves the sheet over the die to position said one margin in alignment with the clamping plane of the other jaw when the jaws are in the loading position.
 14. The machine defined in claim 13 in which the sheet-carrier bar defines an elongated seat for receiving said one margin of the sheet, and in which the bar is pivotally coupled to the arms to be rotatable for dumping said one margin from the seat.
 15. A machine for stretch forming a metal sheet over a die, comprising: a base; a pair of upright frames mounted on the base; each of the upright frames having a means for supporting the die in a fixed relation to the frames and having a gripping means adapted for clamping engagement with the metal sheet; at least one of the gripping means being movably mounted on the associated frame and having coupled thereto a power means connected between the gripping means and the frame for driving the movable jaw over a fixed path with respect to the frame to stretch the metal sheet over the die; the die, gripping means and power means being supported on the frames out of direct contact with the base whereby action and reaction forces occurring during stretch forming are isolated from the base; at least one of the frames and associated gripping means, die-supporting means, and power means being movable as a unit with respect to the base for setup positioning without affecting relative positioning of said one frame and said three means.
 16. The machine defined in claim 17 and further comprising a second machine of identical definition, the machines being positioned adjacent each other, and means for securing together the adjacent gripping means of the several machines to move together when the associated power means are actuated. 